Abstract

Pb isotope analyses, combined with sensitive high-resolution ion-microprobe U-Pb zircon dating, suggest that the giant Broken Hill orebody formed over an ∼6 m.y. period, coincident with a number of small Broken Hill–type deposits lower in the stratigraphic sequence. The formation of the smaller, but significant, Pinnacles deposit was also episodic over ∼7 m.y. and up to ∼10 m.y. prior to the formation of Broken Hill. Using relative ages and stratigraphic position, together with lithologic and isotopic variation, a subsea floor origin for the deposits, including the Broken Hill Pb-Zn-Ag orebody, is proposed. Mn-P–rich banded iron formation stratigraphically above the orebody may have been derived from spent ore-forming fluids exhaled onto the seafloor. Extensive magnetite disseminations in metasedimentary rocks at the same level as the banded iron formation probably represent distal signatures of the same hot springs.